PROJECT SUMMARY Muscular dystrophies are a heterogeneous group of diseases characterized by striated muscle deterioration, resulting in loss of ambulation, respiratory insufficiency, and cardiomyopathy. Heart failure is a common cause of death for muscular dystrophy patients that is becoming an increasingly pressing concern as respiratory therapies allow patients to survive longer and succumb to heart disease. There are no effective therapies for dystrophic cardiomyopathy, stressing the need for novel insights into its progression and treatment. Our preliminary studies have used repeated bolus injections of the ?-adrenergic receptor (?AR) agonist isoproterenol (Iso) to stress the heart in two distinct dystrophic mouse strains. Male mice demonstrate poor survival and widespread cardiomyocyte necrosis in this challenge, while female mice display a remarkable survival benefit and very sparse myocyte necrosis with replacement lesions. These results suggest that female mice resolve initial injury and resist new damage while male mice succumb to their injury. Interestingly, continuous infusion of Iso results in the loss of female resilience, with both sexes displaying precipitous mortality, suggesting that the mechanism of female resilience depends on intermittent adrenergic stress. Iso is nonselective, stimulating both ?1-adrenergic receptors (?1AR) and ?2-adrenergic receptors (?2AR). ?1AR stimulation is proposed to cause the damaging response to Iso in the heart by increasing chronotropy and inotropy, while ?2AR activation has been associated with cardioprotection, likely through coupling to downstream signaling mechanisms distinct from those of ?1AR. These findings inform the central hypothesis that under intermittent adrenergic stress, initial cardiac injury drives female mice to attenuate the detrimental ?1AR-mediated response to Iso and enhance cardioprotective signaling through ?2?R in an estrogen- dependent manner. Furthermore, we hypothesize that the mechanism of ?2?R-mediated protection is lost during continuous stimulation, resulting in the loss of female resilience in that context. To evaluate the roles of ovarian hormones in female resilience, female dystrophic mice will undergo ovariectomy with and without replacement of estradiol (E2) and/or progesterone (P4). Their cardiac injury and survival after repeated bolus injections of Iso will be compared to intact female and male mice to reveal hormone-dependent changes. Next, we propose to measure mouse physiological responses and ?AR expression following repeated Iso injections or continuous infusion, with the purpose of identifying potential alterations in response attenuation and receptor regulation between sexes in each condition. Together, these aims will provide insights into the nature of dystrophic cardiac injury and the mechanism behind the resilience of female dystrophic hearts, while also potentially shedding new light on acquired heart diseases with known sex differences, advancing the long- term goal of identifying a protective pathway that may be clinically targeted to limit injury in patient hearts.